1. Technical Field
Exemplary embodiments relate to amplifiers, more particularly to half bridge pulse width modulation (PWM) amplifiers.
2. Description of the Related Art
Pulse width modulation (PWM) is a modulation technique that varies the width of a pulse or of pulses (e.g., among periodic pulses) according to the magnitude of a modulation signal, and the PWM is widely used in amplifiers (for example, class-D audio amplifiers) and in audio apparatus because the PWM has beneficial characteristics such as high efficiency, high resolution and low power consumption.
In general, a PWM-type audio apparatus carries audio signals with PWM signals having a higher frequency than a frequency of a sampling rate of the audio signals. PWM audio amplifiers convert analog audio signals into digital PWM signals and amplifies the PWM signals to be transmitted to a speaker or a headphone.
Full-bridge PWM amplifiers are commonly used for driving speakers, and half-bridge PWM amplifiers are generally used for driving low-power devices such as headphone which receives input signals with respect to a ground level having zero voltage.
Common two-level PWM amplifiers drive load devices such as headphones to two levels, and the two levels correspond to a predetermined positive level and a ground level or a predetermined positive level and a predetermined negative level according to the PWM signals.
The two-level PWM amplifiers can consume current during an inactive audio signal in addition to dynamic current consumed by the active audio signal. Thus two-level PWM amplifiers have a relatively poor power efficiency because output stage continues switching while maintaining 0.5 (50:50) duty ratio even when the audio input signal level is zero. In addition, the two-level PWM amplifiers have a distortion problem in recovered audio signals due to mismatch of power supply voltages of the output stage.
FIG. 1 is a graph of a waveform illustrating an example of an analog amplifier input signal.
FIG. 2 (FIGS. 2a, 2b, and 2c) illustrates a conventional two-level PWM signal with respect to the amplifier input signal of FIG. 1, however the time-scales in FIG. 1 and FIG. 2 are different.
As illustrated in FIG. 1, the amplifier input signal swings between a positive peak voltage MAX and a negative peak voltage MIN. In addition, the conventional two-level PWM signal is a pulse signal having two levels VDD and VSS.
While the amplifier input signal corresponds to the positive peak voltage MAX, the two-level PWM signal has a maximum period at the VDD level and while the amplifier input signal corresponds to the negative peak voltage MIN, the two-level PWM signal has a maximum period at the VSS level. While the analog amplifier input signal is zero (‘0’), the two-level PWM signal has 0.5 (“50:50”) duty ratio having same high (VDD) width and low (VSS) width. When the analog amplifier input signal increases from zero towards to the positive peak voltage MAX, the two-level PWM signal has a gradually increasing width at the VDD level (and a gradually decreasing width at the VSS level), and when the amplifier input signal decreases from zero towards to the negative peak voltage MIN, the two-level PWM signal has a gradually increasing width at the VSS level (a gradually decreasing width at the VDD level).
The conventional two-level PWM amplifiers (i.e., class-D amplifiers) have a relatively higher efficiency than class-A, class-B and class-AB amplifiers. However, the conventional two-level PWM amplifiers consume current because of continuous switching between VDD level and VSS level (or ground level) even while the analog amplifier input signal is zero (‘0’).